He relocated to Chicago with his family at age 10 and continued training and competition with the Illinois Shotokan Karate Clubs.

At age 15 Fonseca represented Team USA as a junior, competing in International competitions under WUKO (World Union of Karate Organization) which later became WKF (World Karate Federation).

At age 18 he joined the senior US Team and competed in his first World Championship in Malaysia.

In 1996, Fonseca won a bronze medal in the open weight kumite category at the WKF World Championships in Sun City, South Africa, a feat he repeated in 2004 in the -80 kg kumite division at the WKF World Championships in Monterrey, Mexico.

Fonseca was USA Team captain from 2000-2008 and continued his karate training under the coaching of Jack Pressman, James Blann, Tokey Hill, Jo Mirza, and Seiji Nishimura.